Lastly, when k > 3, we will show that Eq. (1.1) has no positive integer solutions but the equation x2 − kxy + y2 − x = 0
has positive integer solutions. Moreover, we will show that the equations x2 −kxy−y2 ∓x = 0 and x2 −kxy−y2 ∓y = 0
have positive solutions when k ≥ 1.
Solutions of some of the above equations are related to the Fibonacci numbers. Now we briefly mention the Fibonacci
sequence {Fn}. The Fibonacci sequence {Fn} is defined by F0 = 0, F1 = F2 = 1 and Fn = Fn−1+Fn−2 for n ≥ 3. Fn is called the
nth Fibonacci number. Fibonacci numbers for negative subscripts are defined as F−n = (−1)nFn for n ≥ 1. It is well known
that Fn+1 = Fn + Fn−1 for every n ∈ Z. For more information about Fibonacci sequence one can consult [2,3]. Let α and β
denote the roots of the equation x2 −x−1 = 0. Then α =
1 +
√
5
/2 and β = (1−
√
5)/2. It can be seen that αβ = −1
and α + β = 1. Moreover it is well known and easy to show that